Separation of a gas mixture

ABSTRACT

A method is described for use in the separation of carbon dioxide from a gas mixture comprising carbon dioxide. The method includes the steps of: (i) compressing and cooling the gas mixture using a compressor to form a two-phase mixture including liquid carbon dioxide (ii) separating a liquid carbon dioxide stream from the two-phase mixture; and (iii) recirculating at least a part of the liquid carbon dioxide stream and introducing the recirculated liquid stream into a process stream by recirculating separated liquid CO 2  into an upstream process stream, cooling of the process stream can be obtained. By using the liquid stream, additional cooling is possible as cooling is affected by the evaporation of the liquid CO 2 . Thus the recirculated liquid can be used to reduce the temperature of the process stream.

The present invention relates to the separation of a gas mixture. Aspects of the invention provide separation of a component from a mixture of gases. Aspects of the invention relate to the separation of a relatively more condensable gas from a mixture in which it is mixed with one or more relatively less condensable gases. In particular, but not exclusively, aspects of the invention relate to the separation of carbon dioxide from a stream comprising carbon oxide(s). In some examples, the stream further includes hydrogen.

In examples described below, a method is provided for use in the separation of carbon dioxide from a feed stream comprising carbon oxide(s) and hydrogen.

For environmental reasons it is becoming increasingly desirable to separate gases considered to be greenhouse gases or pollutants from those gases which are environmentally more benign, such as hydrogen, nitrogen or oxygen. In particular, increasing attention is being given to strategies in which the greenhouse gas, carbon dioxide, the principle carbon containing product of the combustion of hydrocarbon fuels, may be separated from a gas stream. Such separated carbon dioxide may subsequently be stored, for example underground in rock formations.

In other cases, it is desirable to purify gases in order to make them fit for certain applications: for example gases such as carbon monoxide, methane, ethane and natural gas are required to have a purity above a particular threshold for use in certain applications. There exist other situations where it is desirable to separate gases on a large scale.

In International Patent Application No. WO2010/012981 there is described a process for separating condensable carbon dioxide from gas mixtures principally including non-condensable hydrogen. The process described comprises in general terms first compressing and cooling the mixture to a pressure and temperature at which carbon dioxide is liquid and thereafter separating the liquid carbon dioxide from non-condensable gases. Thereafter the separated components are adjusted in temperature and pressure by use of a series of heat exchangers and expanders integrated amongst themselves and with those used to cool the feed stream to the process so that the total energy across the whole process can be managed for efficiency. The application describes the use of compact, diffusion-bonded heat exchangers to simplify the demands on the hardware needed.

Whilst the process described in WO2010/012981 can provide separation of carbon dioxide from hydrogen-rich gases, there would be benefit in further improving efficiency and/or product purity.

According to a first aspect of the present invention there is provided a method for use in the separation of carbon dioxide from a gas mixture comprising carbon dioxide, the method comprising the steps of: (i) compressing and cooling the gas mixture using a compressor to form a two-phase mixture including liquid carbon dioxide (ii) separating a liquid carbon dioxide stream from the two-phase mixture; and (iii) recirculating at least a part of the liquid carbon dioxide stream and introducing the recirculated liquid stream into a process stream

By recirculating separated liquid CO₂ into an upstream process stream, cooling of the process stream can be obtained. By using the liquid stream, additional cooling is possible as cooling is effected by the evaporation of the liquid CO₂. Thus the recirculated liquid can be used to reduce the temperature of the process stream.

It is envisaged that the liquid CO₂ stream may be introduced at any region of the system at which the cooling is required. The CO₂ liquid stream may include a plurality of sub-streams, each sub-stream may be directed to a region of system.

Preferably the recirculated CO₂ liquid stream is introduced upstream of the compressor. By introducing the recirculated stream upstream of the compressor, for example to the inlet of a compressor, additional advantages can be obtained. For example, by recirculating CO₂ in liquid form, the compression power of the compressor can be reduced. Also, by recirculating CO₂, the feed flow through the compressor can be increased by the addition of the CO₂ stream. Thus problems associated with low flow through a compressor and/or variable flow through a compressor can be reduced or eliminated. If the feed flow through a compressor is reduced, CO₂ can be recirculated to ensure sufficient compressor flow. Thus the system can remain operational even when the feed flow is reduced to what may otherwise be below the operating flow rate for components of the system. Thus the recirculation is of particular benefit during start-up, shut-down and/or during periods where the gas flow rate of the system is insufficient. By increasing the flow through the compressor by using recirculated product stream, a potential problem of compressors of “compressor surge” can be reduced or eliminated. Compressor surge can occur if flow rates through the compressor fall too low and can cause an abrupt reversal of the airflow through the unit, as the pumping action of the aerofoils stalls.

A gaseous stream is preferably fed to the compressor and at least a part of the recirculated liquid carbon dioxide stream is preferably introduced into the gaseous stream, such that the liquid carbon dioxide evaporates before entering the compressor.

The gaseous stream may be for example the feed stream of the compressor, but can be any gaseous stream that is suitable for ensuring that the carbon dioxide evaporates, and preferably mixes efficiently, when being introduced into the gaseous stream and before the mixed stream reaches the compressor inlet. For example where the gas mixture comprises syngas, the gaseous stream may be a hydrogen rich gas stream, or a synthesis gas stream.

When the gaseous stream is a hydrogen rich gas stream, the said stream may at least partly be derived from a hydrogen rich gas stream from which the liquid CO₂ is separated in a gas-liquid separator vessel.

According to an alternative embodiment, the entire liquid carbon dioxide stream that is taken from the gas-liquid separator vessel is recirculated indirectly, or directly, to upstream of the compressor. In such cases, preferably the entire liquid carbon dioxide stream is first evaporated in the gaseous stream before being recirculated to the compressor.

Preferably the liquid carbon dioxide is at a temperature of above −56 deg C. According to a preferred embodiment of this aspect of the invention, before being introduced into the gaseous stream, the recirculated liquid carbon dioxide is at a temperature of between −40° C. and 70° C. and preferably between 30 and 50° C.; and at a pressure of between 1 and 20 Mpa, preferably between 10 and 15 MPa. Preferably the temperature is such that the carbon dioxide cools on expansion. Preferably the carbon dioxide is substantially at ambient temperature.

The gas mixture may further include hydrogen, the two phase mixture comprising liquid carbon dioxide and a hydrogen rich gas, wherein the hydrogen rich gas is separated from the two-phase mixture and at least a part of the separated hydrogen rich gas stream is recirculated to the compressor.

The gaseous stream, which is preferably a hydrogen rich gas stream, is preferably the same hydrogen rich gas stream that is separated from the carbon dioxide stream.

In some arrangements, the entire hydrogen rich gas stream that is taken from the separator is recirculated indirectly, or directly, to the compressor.

The gaseous stream is preferably at a temperature of between 10 and 70° C., preferably between 30 and 50° C.; and at a pressure of between 0.5 and 15 MPa, preferably between 1 and 12 MPa. The stream may be at substantially ambient temperature. In some examples, the pressure may be between 3 and 20 MPa, for example between 3.5 and 12 MPa, preferably between 3.5 and 5.5 MPa.

The gaseous stream that is fed to the compressor may be a hydrogen rich gas stream.

Preferably the recirculated liquid carbon dioxide is sprayed into the gaseous stream. The recirculated liquid carbon dioxide may be introduced into the gaseous stream by any suitable method. Advantageously the method of introduction is suitable for achieving a sufficient level of evaporation of the liquid carbon dioxide in the gaseous stream before the carbon dioxide reaches the compressor. A sufficient level of evaporation is where the carbon dioxide is in a suitable state/phase/droplet size distribution for being fed to the compressor.

The recirculated liquid carbon dioxide may be sprayed into the gaseous stream using an atomising nozzle.

In order to achieve this said droplet size, any suitable method known to those skilled in the art may be used. For example, a nozzle can be used to introduce the liquid carbon dioxide into the gaseous stream, suitable nozzles include atomising nozzles, such as liquid-only spray-type nozzles or gas-induced atomising nozzles, where gas is used to assist in the injection of the liquid.

The recirculated liquid carbon dioxide may be sprayed into the gaseous stream using a venturi nozzle.

Preferably the flow path from the introduction of the carbon dioxide to the inlet of the compressor is such that substantially all of the liquid carbon dioxide has evaporated upstream of the compressor inlet. Preferably the length of the flow path is such that evaporation is substantially complete upstream of the compressor. Other features can be provided to increase the rate of evaporation. For example a formation for increasing turbulent flow in the nozzle and/or the flow path, can be provided.

In some arrangements the recirculated liquid carbon dioxide may be sprayed into a pipe, that is preferably at least 2 m in length. In examples, the pipe may be for example of the order of 3 m in flow length. The pipe may have a serpentine configuration.

The particle size of the liquid carbon dioxide entering into the gaseous stream may be less than 200 μm.

The applicants have found that the degree of evaporation of the liquid carbon dioxide in the gaseous stream is especially high when a small particle size of sprayed particles is used. The particle size of the liquid carbon dioxide droplets is preferably less than 200 μm and even more preferably when the droplets are 150 μm or less. In the example below, the droplet size of the sprayed particles is not more than 150 μm. Preferably at least 90%, preferably at least 95%, preferably at least 99% of the droplets have a size less of 150 microns or less.

In some preferred methods, substantially all of the liquid carbon dioxide stream is introduced into the gaseous stream.

The applicants have also found that a high degree of evaporation could be achieved by introducing the liquid carbon dioxide stream into the gaseous stream, by any of the methods mentioned above, within one or more pipes. Preferably the method of introducing the liquid carbon dioxide includes use of apparatus having a plurality of feed pipes, the method including spraying liquid carbon dioxide into each of the feed pipes. Furthermore, the applicants have found that it was particularly advantageous towards evaporation of the liquid carbon dioxide, when the liquid carbon dioxide was introduced at the bottom of the pipe(s) and where the pipe(s) is/are between 2 m and 4 m in length, and/or where the liquid carbon dioxide flowed at a rate of 3 m/s.

The mixture of the gaseous stream and carbon dioxide within this pipe(s), is preferably at a temperature of less than 0° C. and a pressure of between 0.5 and 15 Mpa, preferably between 2 and 12 MPa.

Once the liquid carbon dioxide has evaporated into the gaseous stream within the pipe(s) the mixture is then passed to a compressor. The compressor may for example discharge the mixed stream at a temperature of above 5° C. and at a pressure of between 1 and 15 Mpa, preferably between 10 and 15 MPa.

According to a preferred example of the present invention, a part of the separated liquid carbon dioxide, may also be added to an additional point of the process. For example, liquid carbon dioxide could also be added to the stream discharged by the first compressor, by any suitable method known to those skilled in the art, but preferably by using one or more of the spray nozzles and/or pipes that are described herein. Therefore, in this case, the gaseous stream that the liquid carbon dioxide is evaporated into will at least partly be the discharged mixture from the first compressor and not the recirculated hydrogen rich gas stream. Once the liquid carbon dioxide has evaporated into this discharged stream, preferably within one or more pipe(s), the new mixture temperature may then be reduced by the order of up to 60° C. and passed to a second compressor. This additional liquid carbon dioxide introduction can be repeated as many times as required.

As mentioned above, the applicants have found that by introducing the liquid carbon dioxide stream into the gaseous stream, they were not only able to benefit from the cooling effect of adding cold liquid carbon dioxide to the process stream, but once the liquid carbon dioxide had evaporated within the gaseous stream, they were also able to benefit from the extra cooling from the latent heat of the liquid carbon dioxide evaporation. This is particularly advantageous in a process that involves compressors, because this extra degree of cooling has the advantage of reducing the compression power of the compressor(s) involved in the process and therefore represents a significant economic advantage compared to recycling warm streams.

When the liquid carbon dioxide is recirculated to two or more compressors arranged in series, the first compressor will benefit not only from the cooling of the carbon dioxide stream but also from an increase in gas flow rate through the compressor. Subsequent downstream compressors can also benefit from additional cooling effect associated with further circulated liquid carbon dioxide introduction to the compressor, which is described herein.

A further aspect of the present invention provides a method for use in the separation of carbon dioxide from a feed stream comprising carbon oxide(s) in an apparatus including a compressor, the method comprising the following steps:

-   (i) compressing and cooling the feed stream to form a two-phase     mixture including carbon dioxide; -   (iii) separating a carbon dioxide stream from the two-phase mixture;     and -   (iii) recirculating at least a part of the carbon dioxide stream to     upstream of the compressor.

Methods described herein can be used in systems where carbon dioxide and/or other product streams are recirculated through the system, for example as in a demonstration or research system.

Additionally, the applicants have identified that the methods of aspects of the present invention can provide advantageous benefits in other applications, for example in the operation of apparatus for separation of carbon dioxide from a mixed gas. For example, as discussed further below, aspects of the invention can be applied to a procedure for “starting-up” a process, for example a process for separating carbon dioxide from a mixed gas, wherein the mixed gas may be for example a carbon oxide(s) and hydrogen feedstock.

Furthermore, the applicants have further identified potential advantageous application of aspects of the invention in methods of operating a carbon dioxide separation system where gas flow rate, during operation of the system drops to a value below optimum operation flow for one or more components of the system, for example below an optimum operation flow for a compressor in the system. For example where “compressor surge” may become an issue for any one or more of the compressors in the process. The applicants have found that by recirculating at least a part of a product stream, the gas flow rate in the system can be increased for example to at least a part, or preferably to the all, of the compressors in that process.

In some examples of the present invention, there is provided a flexible mode of operating a separation process, where the amount of carbon dioxide recirculated to particular components of the system, for example to the compressor, can be controlled depending on a parameter of the system, for example the gas flow rate to one or more of the compressor(s). Such control could be carried out manually or automatically, for example under at least partial control of a electronic controller.

Thus, the invention may further comprise the steps of determining information relating to a parameter of the system, and controlling the recirculation of carbon dioxide on the basis of the determined information.

The control of recirculation may relate for example to the proportion of carbon dioxide which is recirculated compared with that removed from the system, and/or to the location of the introduction of the recirculated carbon dioxide, where there is more than one possible recirculation path in the system.

This feature is of particular importance in some examples and is provided independently. Therefore a further aspect of the invention provides a method for use in a system for the separation of carbon dioxide from a feed stream comprising carbon oxide(s) in an apparatus including a compressor, the method comprising the following steps:

-   (i) compressing and cooling the feed stream to form a two-phase     mixture including carbon dioxide; and -   (ii) separating a carbon dioxide stream from the two-phase mixture;     and recirculating at least a part of the carbon dioxide stream to     upstream of the compressor, wherein the method further includes     determining information relating to a parameter of the system, and     controlling the recirculation of carbon dioxide on the basis of the     determined information, for example information relating to flow     rate of a stream and/or information relating to the compressor surge     of one or more compressors.

Preferably the recirculation of the carbon dioxide is controlled so that a process parameter is maintained within a predetermined value range. For example, the recirculation of the carbon dioxide may be controlled so that the feed flow rate is maintained to within a predetermined set of values of flow rate. Preferably the flow rate or other parameter is maintained at a predetermined value.

The method may further include the steps of: (i) determining the gas flow rate of the process; (ii) controlling the amount of carbon dioxide that is recirculated on the basis of the determined gas flow rate.

The system may include for example a gas flow monitoring device which is arranged to transmit information relating to the gas flow rate to a control device, the control device transmitting control instructions which are used to control the recirculation of the carbon dioxide.

The applicants have identified that when the carbon dioxide stream is in the liquid state, this aspect of the invention is particularly advantageous, as not only does it aid in increasing the gas flow rate to at least a part, preferably all, of the compressors in the process but it also provides a cooling benefit, as the carbon dioxide stream is typically cooler than the stream that it is introduced into.

Furthermore, where the carbon dioxide is in liquid form, this can also provide a further cooling benefit due to the evaporation of the liquid carbon dioxide on contact with the gaseous stream; these two cooling benefits can aid in reducing the compression power of the compressor(s) and so therefore are particularly preferred in some examples.

According to this aspect of the invention, when the gas flow rate to one or more of the compressors is less than 115%, more preferably less than 110% of the of “compressor surge” flow rate, the recirculation is adjusted to increase the recirculation to upstream of that compressor. For example, when the gas flow rate of the compressor is 80% of “compressor surge” flow rate, 35% more preferably 30% of “compressor surge” flow rate equivalent of the carbon dioxide drawn from the separator will be recirculated to the compressor(s); preferably using the methods having one or more of the features described herein.

According to all aspects of the present invention, in addition to the carbon dioxide recirculation described hereinabove, there also may be a local recirculation from the stream discharged from one or more of the compressors, in order to assist in increasing the gas flow rate to the said compressor(s).

The carbon dioxide stream may be recirculated in the liquid state.

The mixed gas may include carbon oxide(s) and hydrogen, and preferably is a synthesis gas stream. In some examples, the gas flow rate of the carbon oxide(s) in hydrogen feed stream is between 40 to 45%.

At least a part of the carbon oxide(s) and hydrogen feed stream that exits the compressor may be recirculated back to the compressor. The recirculated carbon dioxide stream may be split into a plurality of sub streams, each substream being introduced in one or a plurality of introduction points in the process.

The carbon dioxide stream may be recirculated to any one or more points throughout the process, for example the recirculated carbon dioxide may be fed to two different compressors in order to seek to improve cooling and/or improve operation of the compressors, for example to reduce the chance of “compressor surge”.

At least a part of the carbon dioxide stream may be removed from the system. Aspects of the present invention may be applied to systems in which carbon dioxide, for example liquid carbon dioxide can be separated from a mixed gas stream. For example, the applicants have identified potential additional advantages when integrating the present invention process with the process described in International Patent Application No. PCT/GB2009/001810. By integrating aspects of these two processes, the applicants have found that resulting method(s) may assist in the recovery of:

-   -   a hydrogen gas stream which can be used simultaneously or         independently for example:         -   as fuel gas feed e.g. for a combustor of a gas turbine of a             power plant,         -   as a feed to an expander (preferably a turbo expander)             which, due to the expansion of the hydrogen rich vapour             stream, may be used to drive a rotor or shaft of a             compressor and/or to drive the rotor or shaft of an electric             generator, and         -   as internal refrigerant;             and/or     -   a purified liquid CO₂ stream         -   of sufficient purity for e.g. CO₂ sequestration (e.g.             storage in the in the underground strata); and/or use in a             wide range of other applications, e.g. in the food, chemical             and oil and gas industry, and         -   as internal refrigerant.

Thus, the present invention also provides a method for use in a process for separating a synthesis gas stream into a hydrogen rich gaseous stream and a purified liquid carbon dioxide stream in a carbon dioxide condensation plant that comprises a heat exchanger system, a gas-liquid separator vessel, and an expansion system comprising at least one expander.

Also the present invention provides a method according to any of the preceding claims wherein the gas mixture includes hydrogen and a hydrogen rich stream is separated from the gas mixture, wherein at least a part of the hydrogen rich gas stream is fed to an expansion system wherein it is subjected to isentropic expansion in an expander, such that a hydrogen rich gas stream is withdrawn from the expander at reduced temperature and reduced pressures and wherein isentropic expansion of the hydrogen rich gas in the expander generates motive power.

In some examples, a series of expanders can be provided. Where expanders are arranged in series, preferably the cooled stream between the expanders is used to effect heat exchange with one or more other process streams.

The motive power that is generated can advantageously be used to drive a machine that is a component of for example, a carbon dioxide condensation plant and/or for driving an alternator of an electric generator. The machine that is driven by the expander(s) is preferably one or more compressor(s), and/or a pump, for example, for pumping liquid carbon dioxide. Where the expander(s) are used to drive an alternator of an electric generator, the electricity is preferably used to power one or more components of the carbon dioxide condensation plant.

According to an example of the present invention, compression and cooling is required to bring the gas mixture to a two-phase mixture including liquid carbon dioxide. For a high pressure gas mixture, for example including carbon oxide(s) and hydrogen, the mixture is cooled to a temperature in the range of −15 to −55° C. This is preferably performed by passing the gas mixture through a heat exchanger system. Thus the mixture will be passed in heat exchange relationship with at least one coolant stream; a plurality of coolant streams are preferably used. In preferred examples, the coolant streams are preferably “internal” streams which are produced as a part of the process wherein the internal streams are selected from the group consisting of for example cold hydrogen rich gas streams and liquid CO₂ streams. In some cases, the heat exchange system includes one or more external refrigerants. Suitable external refrigerants may include for example include ethane, propanes, propene, ethylene, hydrochlorofluorocarbons (HCFC's), ammonia and/or mixed refrigerants; propane being the preferred external refrigerant.

For optimised heat integration in some systems, the heat exchanger system may comprise both external and internal refrigeration. The combination of internal refrigeration with both cold hydrogen rich vapour streams and liquid carbon dioxide streams together with an external refrigeration may be used. The two-phase mixture including liquid carbon dioxide is preferably at a temperature of about minus 50 degrees C. and a pressure greater than 60 bar, preferably greater than 80 bar, 125 bar, 150 bar or 175 bar.

The two-phase mixture from the heat exchanger system may be passed directly to a gas-liquid separator vessel that is preferably operated at substantially the same pressure as the heat exchanger system. Thus, the pressure drop across the separator vessel is typically in the range of 0.1 to 5 bar, preferably, 0.1 to 1 bar, in particular, 0.1 to 0.5 bar. Accordingly, a high pressure gas (for example a hydrogen rich gas) is withdrawn from at or near the top of the gas-liquid separator vessel and a high pressure liquid carbon dioxide stream is withdrawn from at or near the bottom of the gas-liquid separator vessel.

An advantage of the process of the present invention is that at least 75%, preferably, at least 90%, more preferably, at least 95% of the carbon dioxide can be separated from the gas mixture with the carbon dioxide capture level being dependent upon for example:

-   -   the pressure of the compressed gas mixture,     -   on the temperature of the cooled gas mixture

Carbon dioxide capture level generally increases with increasing pressure and reduced temperature.

Where the gas mixture is syngas, typically, at least 98%, preferably, at least 99%, more preferably, at least 99.5%, in particular, at least 99.8% of the hydrogen is recovered in the hydrogen rich gas stream in some examples.

In a process, such as that used for separating carbon dioxide from a mixed gas for example a feedstock comprising carbon oxide(s) and hydrogen, the applicants have found that it is possible to reduce the temperature of one or more of the process streams by introducing at least a part of the separated liquid carbon dioxide into the/those said stream(s). The liquid carbon dioxide can be used in some examples as an alternative, or in addition, to an external refrigerant within the separation process.

The temperature of the liquid carbon dioxide stream is preferably kept above a value where solid carbon dioxide will form. This typically occurs at a temperature of −56° C. (where the triple point for pure carbon dioxide is at 5.18 bar and a temperature of −56.4 C) although the presence of hydrogen may depress this freezing point.

According to the invention there is also provided apparatus for carrying out any of the method features described herein.

The invention also provides apparatus for use in the separation of carbon dioxide from a gas mixture comprising carbon dioxide, the apparatus including:

-   (i) a compressor and heat exchanger for compressing and cooling the     gas mixture to form a two-phase mixture including liquid carbon     dioxide -   (ii) a separator for separating a liquid carbon dioxide stream from     the two-phase mixture; and -   (iii) a recirculation path, for recirculating at least a part of the     liquid carbon dioxide stream from the separator and arranged for     introducing the recirculated liquid stream into a process stream.

Preferably the recirculation path is arranged for introducing the recirculated liquid stream into a process stream upstream of the compressor.

The apparatus may further include a spray device for spraying recirculated liquid carbon dioxide into a process stream.

The apparatus may further include a sensor for determining information relating to a parameter of the system, and a control device for controlling the recirculation of carbon dioxide on the basis of the determined information. The sensor may comprise a flow rate sensor for determining information relating to the flow rate of a process stream.

The invention also provides apparatus for use in the separation of carbon dioxide from a feed stream comprising carbon oxide(s) the apparatus including:

-   (i) a compressor and heat exchanger for compressing and cooling the     feed stream to form a two-phase mixture including carbon dioxide;     and -   (ii) a separator for separating a carbon dioxide stream from the     two-phase mixture; and -   (iii) a recirculation path for recirculating at least a part of the     carbon dioxide stream from the separator to upstream of the     compressor.

The invention also provides apparatus for use in a system for the separation of carbon dioxide from a feed stream comprising carbon oxide(s), the apparatus including:

-   (i) a compressor and heat exchanger for compressing and cooling the     feed stream to form a two-phase mixture including carbon dioxide;     and -   (ii) a separator for separating a carbon dioxide stream from the     two-phase mixture; and -   (iii) a recirculation path for recirculating at least a part of the     carbon dioxide stream from the separator to upstream of the     compressor -   (iv) a sensor for determining information relating to a parameter of     the system, and -   (v) a control device for controlling the recirculation of carbon     dioxide on the basis of the determined information.

It will be understood that features described above in relation to one aspect of the invention may be provided in relation to other aspects in any appropriate combination. For example, features of method aspects may be applied to apparatus aspects and vice versa.

Also provided by the invention is a method and/or apparatus being substantially as herein described optionally having reference to one or more of the accompanying drawings.

In order that the invention may be more readily understood, embodiments of aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows schematically the general features of an example of an arrangement in which separated liquid carbon dioxide is introduced into a process stream;

FIG. 2 shows a process flow diagram of an example having the general arrangement of FIG. 1;

FIG. 3 shows schematically an example of apparatus for use in the introduction of liquid carbon dioxide;

FIG. 4 shows an example of a liquid carbon dioxide spray device for example for use in an apparatus shown in FIG. 3.

In the examples below, the feed stream includes carbon oxide(s) and hydrogen. It will be appreciated however that other feed streams may be used in the systems and methods described.

A carbon oxide(s) and hydrogen feed stream, preferably synthesis gas, may for example be generated from a solid fuel such as petroleum, coke or coal in a gasifier or from a gaseous hydrocarbon feedstock in a reformer. The carbon oxide(s) and hydrogen feed stream obtained from a gasifier, or reformer, may contain high amounts of carbon monoxide. Accordingly, depending on the desired composition of the hydrogen rich gas stream, the carbon oxide(s) and hydrogen feed stream may be treated in a shift converter unit where substantially all of the carbon monoxide contained in the synthesis gas stream is converted to carbon dioxide over a shift catalyst according to the water gas shift reaction (WGSR)

CO+H₂O

CO₂+H₂.

Where the carbon oxide(s) and hydrogen feed stream is of sufficiently high carbon dioxide content, the shift conversion step may be omitted, in which case the carbon oxide(s) and hydrogen feed stream comprises primarily hydrogen, carbon dioxide, carbon monoxide, and steam and minor amounts of methane.

Typically, the carbon oxide(s) and hydrogen feed stream is cooled to a temperature in the range of 30 to 50° C., for example, about 40° C., upstream of the compressor(s), by using a heat exchange with at least one cold process stream, which is used to condense out a predominantly water condensate. Typically, the cold process stream is a process stream used during the generation of the carbon oxide(s) and hydrogen feed stream. The condensate is then separated from the cooled carbon oxide(s) and hydrogen feed stream, for example, in a condensate drum.

Where the carbon oxide(s) and hydrogen feed stream is formed by gasification of petroleum coke or coal in a gasifier, the carbon oxide(s) and hydrogen feed stream that exits the gasifier will also comprise minor amounts of hydrogen sulfide (H₂S) as an impurity (for example, sour synthesis gas). The H₂S impurity is formed by the reaction of COS with steam in the shift converter unit. This H₂S may be captured upstream of the compressor(s), for example, by selectively absorbing the H₂S from the sour carbon oxide(s) and hydrogen feed stream in an absorption tower. Typically, Selexol™ (a mixture of dimethyl ethers of polyethylene glycol) may be employed as the absorbent. Any H₂S that is captured may either be converted into elemental sulphur, using the Claus Process, or into industrial strength sulphuric acid. An alternative system, for example a biological-based system, for example the Paques apparatus of Shell, may be used to remove H₂S.

Alternatively, the sour carbon oxide(s) and hydrogen feed stream may be fed to the compressor of the present invention, where a major portion of the H₂S partitions into the liquid carbon dioxide phase and may therefore be subsequently removed from the CO₂ if required, or processed and/or sequestered with the CO₂, if required. Typically, greater than 95% of the H₂S that was contained in the carbon oxide(s) and hydrogen feed stream partitions into the liquid carbon dioxide phase. Any residual H₂S in the final hydrogen rich gas stream may be removed downstream of the compressor by passing the final hydrogen rich gas stream through an adsorbent bed, for example, a zinc oxide bed, or by passing the final hydrogen rich vapour stream through a scrubber that utilises a suitable liquid absorbent. There is minimal pressure drop, for example, a pressure drop of less than 0.5 bar across the absorbent bed.

After removal of any condensate (see above), the carbon oxide(s) and hydrogen feed stream is preferably dried prior to being passed to the compressor(s), as any moisture in the synthesis gas will freeze and potentially cause blockages in the plant. The carbon oxide(s) and hydrogen feed stream may be dried by being passed through a molecular sieve bed, or an absorption tower that employs a solvent, for example, triethylene glycol, to selectively absorb the water. Preferably, the dried carbon oxide(s) and hydrogen feed stream has a water content of less than 1 ppm (on a molar basis).

Typically, the dried carbon oxide(s) and hydrogen feed stream comprises at least 40 mole % hydrogen, preferably, at least 50 mole % hydrogen, in particular 55 to 60 mole % hydrogen. It may also comprise at least 30 mole % carbon dioxide, for example at least 35 mol % carbon dioxide. Even if it is not preferred, carbon monoxide can be tolerated in the carbon oxide(s) and hydrogen feed stream treated according to the present invention, e.g. if the WGSR is only partial.

Usually, prior to being compressed during and/or after the WGSR, the carbon oxide(s) and hydrogen feed stream is at a pressure in the range 1 to 12 MPa.

FIG. 1 shows schematically the general features of an example of an arrangement in which separated liquid carbon dioxide is introduced into a process stream.

As shown in FIG. 1, a feed stream 100 comprises a gas mixture including carbon oxide(s), CO_((x)) and hydrogen H₂. Such a feed stream may be a syngas stream for example produced by a water gas shift reaction, or by other means. It will be understood that features of the present invention may be applied in relation to other feed streams, in particular other streams including carbon dioxide.

The feed stream 100 is first fed to a compressor 102, which pressurises the gas mixture before it is fed to a cooling device 104, where the gas mixture is cooled such that a two-phase mixture 106 is formed, including a liquid phase comprising CO₂ and a gas phase. In the present example, the gas phase may be hydrogen-rich, but it will be understood that the composition of the gas phase will depend on the initial composition of the gas mixture. It will further be understood, and as indicated below, the compression and/or cooling may be carried out by a series of compressors and/or cooling devices, and may be carried out in any appropriate order. Here, the compression is carried out prior to the cooling.

The two-phase mixture 106 is then fed to a separation device 108 at which the mixture is separated into a separate CO₂ liquid stream 110 and a H₂-rich stream 112. The CO₂ liquid stream 110 can be removed via path 114, and/or can be recirculated, for example here via CO₂ return path 116 to upstream of the compressor 102. The CO₂ from the CO₂ liquid stream 110 is passed into the feed stream 100, and the evaporation of the CO₂ liquid provides additional cooling to the system.

Also, by controlling the return of CO₂ along the return paths, the feed flow into the compressor 102 can be maintained to a required value, even in situations where the flow of the feed into the system may be variable and/or reduced.

In the present example, at least a portion of the H₂-rich stream 112 is also recirculated via H₂ return path 118 to upstream of the compressor 102. In this case, and for example as described further below, the returned CO₂ liquid stream 116 and returned H₂ stream are both introduced into the feed stream 100. Thus further control of the flow into the compressors can be achieved. Also, the composition of the feed stream 100 can be manipulated to for example increase the amount of H₂ and/or CO₂ in the feed stream 100, if desirable. Also, it is possible to provide recirculation within the system, for example on start-up, shutdown or any other appropriate time, for example during testing or demonstration procedures. It is envisaged that a system may be arranged so that substantially all of the H₂ and CO₂ is recirculated.

FIG. 2 shows a process flow diagram for one example of the present invention having the general configuration of a system of FIG. 1. Referring to FIG. 2, a synthesis gas stream 1 is provided as a feed stream. The synthesis gas feed stream 1 contains in this example 56.9 mol % H₂, 41.4 mol % CO₂, 1.2 mol % CO and trace amounts of CH₄, Ar and N₂. It will be appreciated that feed streams having other compositions can be used. The feed stream 1 may for example be free of hydrogen sulphide or may contain hydrogen sulphide, in which case, the hydrogen sulphide will condense out of the synthesis gas feed stream together with the CO₂ as described in more detail below. It will be appreciated that the system and method described can be used for the separation of CO₂ from compositions other than syngas; other gas mixtures could be used as the feed stream as appropriate.

A recirculated liquid CO₂ stream 78 and H₂ stream 76 are introduced into the feed stream 1 as described in more detail below and provide initial cooling of the feed stream from a temperature of about 40° C. to −12° C. (100% Recycle) at a pressure of 73 Bar. The evaporation of the liquid CO₂ into the feed stream 1 provides significant cooling.

The resulting cooled synthesis gas steam 3 is fed to a first compressor 5 of a compression system. The compression system further comprises a second compressor 11, the two compressors 5 and 11 being arranged in series.

Gas stream 7 exits the first compressor 5 at a pressure of 130 bar and a temperature of 32.4° C., the increase in temperature arises from heat of compression.

In order to remove heat of compression from gas stream 7, a further recirculated liquid CO₂ stream 80 is introduced into synthesis gas stream 7, giving a cooler gas stream 9 that is at a pressure of 129 bar and a temperature of 27.6° C. Gas stream 9 is then sent to the second compressor 11.

Gas stream 13 exits the second compressor 11 at a pressure of 175 bar and a temperature of 57.9° C. before being cooled by an external coolant in a first heat exchanger 15. In accordance with usual practice, the system is arranged such that the pressure drop across the first heat exchanger 15 is kept to a minimum, the emerging gas stream 17 being at a pressure of 174 bar and a temperature of 40.0° C.

The high pressure gas stream is then fed to a cooling system. In this example, the cooling system includes an external heat exchanger E-105 employing an external refrigerant, for example propane and an internal heat exchanger E-106 comprising a multi-channel heat exchanger employing internal process streams. In this example as shown in FIG. 2, the compressed stream 17 is split into two substreams 17′ and 17″. Stream 17′ is cooled to form a two-phase mixture 25 across a single external heat exchanger E-105. Substream 17″ is passed through a multichannel heat exchanger E-106 where it is cooled against cool internal process streams including liquid CO2 stream 41 as discussed below. The cooled stream is combined with cooled stream 25 to form a single multi-phase stream 27.

The cooling arrangement of FIG. 2 is given only as an example and it will be understood that other cooling arrangements are possible using external and/or internal cooling. For example, the stream may be cooled as a single stream without splitting or split into additional sub-streams, each sub-stream being cooled according to a different cooling path. In some arrangements, control of the proportion of the stream being split into each cooling path can be effected to give greater control over the cooling of the stream.

The resulting low temperature multiphase stream 27 comprises a liquid phase and a gaseous phase and in this example has a vapour fraction of 65.6 mol %.

The low temperature multiphase stream 27 is fed at a pressure of 173 bar and a temperature of −27° C. to a first gas-liquid separator vessel 29. A H₂-rich gas stream 30 is withdrawn from the top of the gas-liquid separator vessel 29, while a CO₂ liquid stream 41 is withdrawn from the bottom of the gas-liquid separator vessel 29.

The CO₂ liquid stream 41 comprises more than 97 mol % CO₂ with H₂ and trace amounts of CO, CH₄, Ar and N₂. The CO₂ liquid stream 41 may be of sufficient purity for export purposes.

As appropriate, further separations may be effected, for example by feeding the CO2 liquid stream 41 to one or more further separators, with additional cooling being provided as necessary. In some examples, the resulting liquid CO2 streams may be combined to form a single CO2 liquid product stream.

Some or all of the CO₂ liquid stream 41 is then optionally passed through the multi-channel heat exchanger E-106 to serve as an internal coolant of gas stream 17″. A valve 28 is provided to control the proportion of the CO2 stream 41 entering the heat exchanger E-106.

Where the CO2 stream has been split, it is recombined downstream of the heat exchanger E-106 to form a single CO2 stream 74.

Combined CO₂ liquid stream 74 may be at a temperature for example of 48.8° C. A part or all of the liquid CO₂ stream may then be removed from the system for subsequent use and/or storage. CO₂ liquid which is not removed, is then recirculated through the system for example as now described. At least a part of the recirculated CO₂ is preferably used as a coolant upstream.

The liquid CO₂ stream to be recirculated is split into two sub-streams, upstream liquid CO₂ stream 78 and downstream liquid CO₂ stream 80. Depending on the recirculation to be carried out, for example the location and nature of cooling to be delivered to the system, flow can be split as desired through the streams. The splitting of the stream may be fixed, or may be variable, for example in dependence on a parameter of the system.

The upstream liquid CO₂ sub-stream 78 is introduced into the feed stream 1 upstream of the first compressor 5. In examples, the CO₂ will be at a temperature less than that of the feed stream 1 and will therefore provide cooling. In addition, the evaporation of the liquid CO₂ provides significant additional cooling compared with the introduction of gaseous CO₂; by using the latent heat of evaporation to provide additional cooling, heat efficiencies in the system may be achieved.

The downstream CO₂ sub-stream 80 is introduced into the stream downstream of the first compressor 5 and upstream of the second compressor 11. Thus further cooling is provided between the two compressors, which can remove at least a part of the heat of compression.

In some examples, some or all of the H₂ rich gas withdrawn from the separator 29 may be extracted directly from the system. Preferably however, the H₂ rich gas stream is further managed within the system to recover temperature and/or pressure of the stream. An example of such a heat and pressure management system is described in relation to FIG. 2.

The H₂-rich gas stream 30 may be is split into separate streams which are subject to separate processing. In the example of FIG. 2, however, the hydrogen is retained as a single stream.

The H₂-rich gas stream 30 is passed to an expander 44 where it is subject to expansion, thus decreasing the pressure and temperature of the stream. The expander 44 preferably comprises a turbine which is used to recover work. The expanded stream is then fed through a first set of channels in the multi-channel heat exchanger E-106, wherein the stream exchanges heat with other process streams, preferably by counter-flowing internal process streams in the other set of channels, in this case cooling the gas stream 17″.

H₂-rich gas stream 39 exits the set of channels of the multi-channel heat exchanger E-106 and is passed to a second expander 45, where it is expanded to lower pressure. Stream 42 exits the expander 45 for example at a pressure of 74.0 bar and a temperature of 40° C. and is passed to a further set of channels of the multi-channel heat exchanger E-106 where it exchanges heat with other internal process streams, to form H₂ rich vapour stream 43.

Passing the H₂ streams through the expanders provides cooling and also can recover work. For example, the expanders may include turbines.

H₂ rich gas from stream 43 may then be removed from the system for storage or directly for further use. For example, the H₂-rich gas may be passed to a Power Island (not shown) for example to be used as a component of a fuel gas feed for the combustors of a gas turbine. The H₂ rich gas may be combined with other components, for example may be diluted with medium pressure N₂ and/or steam.

At least a part of the resulting H₂ rich gas stream 76 may then be recirculated and introduced into the feed stream 1. In this example, the H₂ is circulated to a region upstream of both compressors 5 and 11.

Recirculation of some or all of the H₂ rich stream may be desirable for example on start-up or shut down of the system or at other times for example in view of system operation issues. It may be advantageous to operate the system using recirculation of the H₂ rich stream (and/or the CO₂ stream) for example during testing or demonstration procedures. Recirculation of part or all of the H₂ stream may also be used to control or vary the composition of the feed stream.

The recirculation of the H₂ and/or CO₂ streams is preferably controllable as discussed further below.

The example described above and illustrated in FIG. 2 is only an illustration of aspects of the invention. Features of the invention may be applied to any appropriate arrangement. For example, different numbers and/or arrangements of heat exchangers, separators, compressors and/or expanders can be used. Some of those components may be omitted. For example, while two expanders 44 and 45 are shown, the number of expanders can be increased or decreased. Preferably there are at least two expanders with heat exchange with other internal streams being effected between expanders. The person skilled in the art will also understand that the operating pressure and temperature of the expanders can be varied. In some arrangements, it will be preferred that the pressure is dropped through the expander(s) to a pressure of at least 30 bar. The expanders 44 and 45 may be connected to electric motors to recover energy, for example in the form of electricity. The electricity may be either used in the process or exported from the process. Alternatively, and as shown schematically in FIG. 2, the expanders may be directly coupled to one or more of the compressors (5 and 11 in FIG. 2). This may be effected for example by mounting the expander(s) and compressor(s) on a common shaft so that the isentropic expansion of the hydrogen rich vapour in the expander(s) is used to turn the common shaft and to drive the compressor(s). Accordingly, the net power consumption for the flow scheme of FIG. 2, may be for example 24.38 MW. A steam turbine ST-101 is present in this example to provide the additional power required to drive the compressors 5 and 11.

Operation of System

Various examples of modes of operation of the system will now be described:

Start-Up

When the system is first started up, the system is configured such that substantially all of the gases in the system—including, as the system begins effective operation, separated H₂ rich gas and CO₂— are recirculated via paths 76, 78 and 80 within the system. During initial start-up mode, preferably no components are exported from the system.

As the system begins to effect separation of the mixed gases, and as the various components of the system move towards their optimum operation, the proportion of H₂ rich gas and/or CO₂ recirculated is reduced.

When full operation is reached, and the operating temperatures and pressures of the various components of the system are reached, then the recirculation is minimised, or terminated. Alternatively, as discussed further below, some recirculation may be carried out to provide additional cooling in the system.

By using the recirculation during start-up, the amount of CO₂ being released into the atmosphere from the system can be reduced.

In addition to the recirculation of the H₂ rich gas and CO₂ being changed during the start-up procedure, it will be appreciated that the cooling path configuration can also be changed during start-up as the heat exchangers move towards their normal operating temperatures. For example, it will be seen that by diverting the flow through some or all of the various sub-stream paths described above, a selection of different preferred cooling configurations can be used as start-up proceeds.

Shut-Down

In a similar manner to that described above for the Start-up procedure, as the system is moved to shut-down, the recirculation of H₂ rich gas and/or CO₂ can be used to optimise operation and to minimise release of unwanted components into the atmosphere.

System Control

During operation of the system, it may be advantageous to use the recirculation of H₂ rich gas and/or CO₂ to optimise aspects of the system, and/or as a part of the control of system parameters, for example flow rate of one or more streams in the system.

Cooling Control

As described above, by recirculating CO₂ liquid from downstream of the separator to elsewhere in the system, substantial cooling can be obtained, in particular where the liquid is evaporated and introduced into a process stream. By use of recirculated liquid CO₂ to particular regions of the system, cooling can be targeted to those regions. By varying the regions receiving the recirculated CO₂, and/or the amount of CO₂ recirculated, some control of the cooling in the system can be obtained. In the example shown in FIG. 2, the CO₂ is introduced upstream of one or more compressors. In other arrangements, targeted cooling can be directed elsewhere in the system.

Various temperature controllers can be arranged in the system (TC in FIG. 2). The determined temperature at particular regions of the system can be used to control the amount and/or destination of recirculated streams within the system and thus the temperature of regions of the system. The control on the basis of the determined temperature can be carried out for example manually or automatically, for example under computer control.

Flow Rate Control

By controlling the amount and/or destination of recirculated streams in the system, the flow rate of particular streams in the system can be controlled.

For example, having reference to FIG. 2, if the flow rate of the syngas feed stream 1 were to drop during operation, the flow rate of gases to the compressors could be maintained, or the drop could be reduced, by providing recirculated CO₂ liquid 78, 80 upstream of the compressors 5, 11. By maintaining the overall flow rate in the system, or reducing the flow rate drop of process streams in the system, it is possible for the system to continue to operate at normal operating conditions which otherwise may not be possible should the flow rate drop.

For example, by recirculating H₂ and/or CO₂ (liquid or gas) upstream of the compressors, the required flow to the compressors can be maintained. This can avoid surge of the compressor.

Flow controllers (FC) and/or pressure controllers (PC) may be provided in the system to monitor or determine process stream flow and/or pressure at particular locations in the system. In a preferred system, the system further includes control apparatus for receiving information relating to one or more process parameters, for example flow rate, pressure, and controlling the location and amount of recirculated streams in the system on the basis of the received parameters.

For example, a flow controller is arranged to determine the flow rate of the syngas feed stream 1. If the flow controller indicates that the flow rate has dropped below a predetermined value, the amount of CO₂ recirculated as stream 78 to upstream of the compressor 5 is increased. When the flow controller indicates that the flow rate of the feed stream has been restored to its normal value, the recirculation stream 78 can be reduced or even stopped.

FIG. 3 shows schematically an example of apparatus for use in the introduction of liquid carbon dioxide for example in a system shown in FIG. 2. The apparatus includes a first evaporation device 200 and a second evaporation device 202. The first evaporation device 200 is arranged upstream of the first compressor 5 and receives the feed stream 1 (which may include H₂ rich gas depending on whether the H₂ rich gas is recirculated) and also first liquid CO₂ stream 78 and outputs a cooled stream 3 which is fed directly to the compressor 5 inlet.

The liquid CO₂ path 204, feed stream path 206 and cooled stream path 208 are in fluid connection by means of a piping rack 210 including five connector pipes 212 which extend from the CO₂ path 204 to the cooled stream path 208, the feed stream path having fluid connection with each of the five connector pipes 212 part way between the CO₂ path 204 and cooled stream path 208.

At the base of each of the connector pipes 212 and at the interface with the CO₂ path 204 is a CO₂ spray nozzle 214 for spraying atomised CO₂ into the connector pipe 212. In operation, recirculated liquid CO₂ from the CO₂ path 204, for example having a temperature of 40° C. and a pressure of 148 bar, is injected into the compressor suction stream using the CO₂ spray nozzles 214. The atomised CO₂ is then mixed with H₂ rich gas from feed path 206 having a temperature of 40° C. and a pressure of 73 bar, in the piping rack 210 to give a mixed gas having a temperature of −12° C. and a pressure of 73 bar. The mixed gas is passed to the cooled stream path 208 and then to the compressor 5 inlet.

Thus the evaporation of the liquid CO₂ stream provides significant cooling upstream of the compressor 5.

Downstream of the compressor 5, the compressed gas stream 7 is passed to the second evaporation device 202 having a similar structure to the first evaporation device 200. The second evaporation device 202 is arranged upstream of the second compressor 11 and receives the pressurised stream 7 from the first compressor, and also second liquid CO₂ stream 80 and outputs a cooled stream 9 which is fed directly to the compressor 11 inlet.

The liquid CO₂ path 204′, process stream path 206′ and cooled stream path 208′ are in fluid connection by means of a piping rack 210′ including five connector pipes 212′ which extend from the CO₂ path 204′ to the cooled stream path 208′ as for the first apparatus 200. At the base of each of the connector pipes 212′ at the interface with the CO₂ path 204′ is a CO₂ spray nozzle 214′ for spraying atomised CO₂ into the connector pipe 212′. The CO₂ liquid may be sprayed into several pipes simultaneously. In operation, recirculated liquid CO₂ from the CO₂ path 204′, for example having a temperature of 40° C. and a pressure of 148 bar, is injected into the compressor suction stream using the CO₂ spray nozzles 214′. The atomised CO₂ is then mixed with H₂ rich gas from process stream path 206′ having a temperature of 28° C. and a pressure of 112 bar, in the piping rack 210′ to give a mixed gas having a temperature of 6° C. and a pressure of 112 bar. The mixed gas is passed to the cooled stream path 208′ and then to the compressor 11 inlet.

FIG. 4 shows schematically the CO₂ spray nozzle 214 arranged at the base of the connector pipe 212. The liquid CO₂ is injected from the bottom of the connector pipe 212 as small particles 216 having a size of about 150 μm by using atomisation spray nozzles. The small particles 216 evaporate quickly; it is estimated that in some arrangements, the evaporation time was less than one second in view of the heat transfer coefficient in the H₂ rich gas 218 and the atomised CO₂ flow considerations.

The size of the pipe 212 is preferably chosen so that the CO₂ has all evaporated before the mixed gas 220 reaches the compressor inlet. Preferably the piping size is chosen to give a flow velocity of about 3 m/s. In this example, the length of the pipe 212 from the CO₂ path to the cooled gas path 208 is about 3 m which ensures that evaporation is complete before transfer of the mixed gas 220 to the compressor.

The CO₂ spray nozzle 214 may have any appropriate design. Preferably the spray nozzle includes or is enhanced with a downstream piping arrangement or device that creates a turbulent flow to increase turbulence in the CO₂ liquid flow and thus facilitate thorough mixing and sufficient contact time/residence time ensuring complete evaporation.

Using the apparatus described, the spraying, mixing and evaporation of the CO₂ can be achieved. Due to the use of a direct mixture method, evaporation can take place in a relatively simple and compact configuration. By spraying the CO₂ liquid at each compressor inlet, the overall gas temperature throughout the compression cycle can be reduced.

Features of aspects of the invention have been described above by way of example and changes can be made within the scope of the invention. 

1. A method for use in the separation of carbon dioxide from a gas mixture comprising carbon dioxide, the method comprising the steps of: (i) compressing and cooling the gas mixture using a compressor to form a two-phase mixture including liquid carbon dioxide (ii) separating a liquid carbon dioxide stream from the two-phase mixture; and (iii) recirculating at least a part of the liquid carbon dioxide stream and introducing the recirculated liquid stream into a process stream.
 2. A method according to claim 1, wherein the recirculated CO₂ liquid stream is introduced upstream of the compressor.
 3. A method according to claim 1, wherein a gaseous stream is fed to the compressor and at least a part of the recirculated liquid carbon dioxide stream is introduced into the gaseous stream, such that the liquid carbon dioxide evaporates before entering the compressor.
 4. A method according to claim 1 wherein the gas mixture further includes hydrogen, the two phase mixture comprising liquid carbon dioxide and a hydrogen rich gas, wherein the hydrogen rich gas is separated from the two-phase mixture and at least a part of the separated hydrogen rich gas stream is recirculated to the compressor.
 5. A method according to claim 2, wherein the gaseous stream that is fed to the compressor, is a hydrogen rich gas stream.
 6. A method according to claim 2, wherein the recirculated liquid carbon dioxide is sprayed into the stream.
 7. A method according to any claim 2, wherein the particle size of the liquid carbon dioxide entering into the gaseous stream is less than 200 μm.
 8. A method according to claim 1, including the further steps of: determining information relating to a parameter of the system, and controlling the recirculation of carbon dioxide on the basis of the determined information.
 9. A method according to claim 8, wherein the parameter relates to flow rate of a stream and/or to the compressor surge of one or more compressors.
 10. A method according to claim 1, comprising the following steps: (i) determining a gas flow rate of the process; (ii) controlling the amount of carbon dioxide that is recirculated on the basis of the determined gas flow rate.
 11. A method according to claim 1 wherein the recirculated carbon dioxide stream split into a plurality of sub streams, each substream being introduced in one or a plurality of introduction points in the process.
 12. A method according to claim 1, wherein at least a part of the carbon dioxide stream is removed from the system.
 13. A method according to claim 1 wherein the gas mixture includes hydrogen and a hydrogen rich stream is separated from the gas mixture, wherein at least a part of the hydrogen rich gas stream is fed to an expansion system wherein it is subjected to isentroic expansion in an expander, such that a hydrogen rich gas stream is withdrawn from the expander at reduced temperature and reduced pressures and wherein isentropic expansion of the hydrogen rich gas in the expander generates motive power.
 14. A method for use in a system for the separation of carbon dioxide from a feed stream comprising carbon oxide(s) in an apparatus including a compressor, the method comprising the following steps: (i) compressing and cooling the feed stream to form a two-phase mixture including carbon dioxide; and (ii) separating a carbon dioxide stream from the two-phase mixture; and (iii) recirculating at least a part of the carbon dioxide stream to upstream of the compressor, wherein the method further includes: determining information relating to a parameter of the system, and controlling the recirculation of carbon dioxide on the basis of the determined information.
 15. An apparatus for carrying out a method according to claim
 1. 16. An apparatus for use in the separation of carbon dioxide from a gas mixture comprising carbon dioxide, the apparatus including: (i) a compressor and heat exchanger for compressing and cooling the gas mixture to form a two-phase mixture including liquid carbon dioxide (ii) a separator for separating a liquid carbon dioxide stream from the two-phase mixture; and (iii) a recirculation path, for recirculating at least a part of the liquid carbon dioxide stream from the separator and arranged for introducing the recirculated liquid stream into a process stream.
 17. An apparatus according to claim 16, wherein the recirculation path is arranged for introducing the recirculated liquid stream into a process stream upstream of the compressor.
 18. An apparatus according to claim 16, further including a spray device for spraying recirculated liquid carbon dioxide into a process stream.
 19. An apparatus according to claim 16, further including a sensor for determining information relating to a parameter of the system, and a control device for controlling the recirculation of carbon dioxide on the basis of the determined information.
 20. An apparatus according to claim 19 wherein the sensor comprises a flow rate sensor for determining information relating to the flow rate of a process stream. 